1. Field of the Invention
The present invention relates to drive circuitry of the type of used for switching power amplifiers, and in particular to drive circuitry capable of delivering, to a capacitive load, a voltage substantially in the form of a square wave (quasi-square wave) at high frequency and high efficiency.
2. Description of the Prior Art
In recent years not only has the number of different versions of MOSFET devices and MOS-controlled devices increased, but also the applications of such devices have increased. Examples of such devices are insulated-gate bipolar transistors and MOS-controlled thyristors. Each of these devices has the potential of operating at higher frequencies than earlier technology, and each is a voltage-controlled device. The control input of such devices usually has a capacitance associated therewith, and thus appears as a capacitive load to the drive circuit. The control input, for example, may be the gate of an MOS transistor.
For the most efficient operation of these devices as switches, a voltage square wave must be supplied to this input. This means that the drive circuit must deliver pulses of current to alternatingly charge and discharge the capacitance. Prior art drive circuitry of this type have made use of a so-called totem pole, push-pull arrangement to deliver these current pulses.
A drive circuit of this type exhibits losses equal to F.multidot.C.multidot.V.sup.2, wherein F is the operating frequency of the drive circuit, C is the capacitance of the driven device (i.e., the capacitance "seen" by the drive circuit), and V is the peak-to-peak voltage applied to the input of the driven device. At higher operating frequencies, or in applications wherein a number of driven devices are connected in parallel, these losses in the drive circuit can become appreciable.